This invention relates to a signal function generator.
Function generators often generate interfering harmonics that are often associated with the steep signal edges of the generated signal waveform. Harmonics caused by large current changes are particularly troublesome in monolithic integrated circuits if in the presence of a relatively large capacitive load, the associated voltage level is changed within a relatively short time interval. This occurs in digital circuits which change a large number of states simultaneously and synchronously with a common clock. The associated load currents add up on the clock and supply lines and produce electromagnetic interference via these lines. If external loads are switched via output stages, the load currents also appear on the output lines of these output stages resulting in additional electromagnetic interference. The magnitude of the interference is dependent on the output stage, the load, and the kind and length of the interconnecting lines. The contribution of the lines is largely determined by the geometry and is subject to only small variations. In contrast, the contributions of the output stage and the load depend on the respective circuit technology, the manufacturing tolerance, the temperature, the number and size of the transistors, and other variables, and may therefore vary widely.
In CMOS circuits, the load includes the sum of the input capacitances of the transistor gates to be switched. Typical sum values for the input capacitances to be taken into account range between about 5 pF and several hundred pF. Even though it is not generally necessary to cover the entire range of values, it is readily apparent that conventional complementary driver circuits are not suitable for use as function generators, because in conjunction with the connected load capacitances they cause an approximately exponential charging or discharge process. A disadvantage of the resulting switching edge is that during the transition, the waveform has widely differing slopes. At the beginning, the slope is very steep, so that many harmonics are produced; at the end, the waveform rises very slowly, so that the final level is reached only very late. Eliminating both disadvantages simultaneously is not possible with prior-art complementary circuits. To hold the various tolerances for the maximum permissible transition time of the switching edge, the complementary driver circuit is designed for the worst-case tolerance combination, so that the edges at the beginning are much too steep and, thus, produce unwanted harmonics.
There are also other cases where the shape of the switching edges has to be considered, for example if on rising and falling edges of digital signals, the respective switching thresholds are to be reached at particular points of time. This is important, for example, if overlapping or nonoverlapping is required. In the case of other digital signals where only one switching edge is important, for example, only the shape and duration of this edge is of interest. The shape and duration of the second edge is insignificant for interference considerations if it is less steep than the first edge. On the other hand, there are signals whose edges are to be as trapezoidal or symmetric as possible.
The problem of radio-frequency electromagnetic emissions from signal, supply, and clock lines becomes more critical with the increasing complexity of monolithic integrated circuits, the increasing number of transistors, and increasing processing speed. Specifications relating to interference to other equipment or to internal interference are frequently found under the abbreviation EMC (electromagnetic compatibility). Internal interference may occur, for example, if in addition to digital subcircuits, analog subcircuits are present in the respective circuit and signal corruption is caused therein by spurious digital signals being superimposed on the analog signals.
Therefore, there is a need for a function generator that controls the generated waveform in the edge regions of the waveform to reduce the generation of signal noise such as harmonic frequencies.